Medical personnel take care of Ebola patients in a clinic in the Kenema District on the outskirts of Kenema, Sierra Leone, on July 27. / Youssouf Bah, AP

by Liz Szabo and Karen Weintraub, Special for USA TODAY

by Liz Szabo and Karen Weintraub, Special for USA TODAY

The growing Ebola outbreak in West Africa serves as a grim reminder that deadly viruses are only a plane ride away from the USA, health experts say.

The outbreak is the largest and deadliest on record, with more than 670 deaths and more than 1,200 infections in Guinea, Liberia and Sierra Leone, according to the Centers for Disease Control and Prevention. Fatality rates for Ebola have been as high as 90% in past outbreaks, according to the World Health Organization.

The virus - which has an incubation period of a few days to three weeks - could easily travel to the USA through infected travelers, says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

"A case very well could fly out of Africa, only to be detected in some distant country," says Osterholm, who served as an adviser to the George W. Bush administration on bioterrorism.

The CDC on Monday announced that it has sent an alert to health care providers in the USA to help them spot symptoms of the disease.

Health experts at the CDC have been working with African nations since the Ebola outbreak began in March. But officials are on alert now, after news that a man with Ebola was able to board a plane and arrive in Lagos, the capital of Nigeria. He later died.

Two Americans providing humanitarian assistance in West Africa have become infected with Ebola. Family members of one of them, Kent Brantly, a doctor, had been living with him in Africa, but returned to the USA before he began showing symptoms. To be careful, however, the family is on a "21-day fever watch," in which they are being asked to monitor themselves for symptoms, says Stephan Monroe, deputy director of the CDC's national center for emerging and zoonotic infectious diseases.

Yet while Ebola is a fearsome disease, the virus "would not pose a major public health risk" in the USA, Osterholm says.

That's because people need to be in intimate contact to spread the virus, Osterholm says.

Ebola is actually much harder to spread than respiratory infections, such as influenza or measles. Those viruses pose a much greater threat on a plane or in any confined space, says Osterholm, who notes that people cannot spread the Ebola virus simply by sneezing or coughing.

Ebola also can only be spread by people with active symptoms, Monroe says.

"No Ebola cases have been reported in the United States and the likelihood of this outbreak spreading outside of West Africa is very low," says Monroe, who says that the CDC has sent 12 experts to Africa to help with the crisis. "While it's possible that someone could become infected with Ebola in Africa" before boarding a plane to the USA, "it's very unlikely that they would spread it to other passengers."

Ebola does spread readily through body fluids, such as blood and saliva, Osterholm says. On a plane, a sick person could potentially contaminate the bathroom if he or she vomits or has diarrhea.

Stephen Morse, an epidemiologist at the Mailman School of Public Health at Columbia University, says the issue of how Ebola spreads is complex. Sweat and saliva carry much lower levels of the Ebola virus than blood and stool, so the virus spreads less readily through those fluids.

"I don't think we've seen actual cases (passed through contact with sweat or saliva)," Morse says. "There may someday be a strain that's more capable of doing that, but so far it's more theoretical than actual."

Ebola has spread in Africa partly because of religious customs, in which family members wash the bodies of deceased relatives to prepare them for burial.

The virus also has spread to health care workers in Africa, where six or seven patients may share a single hospital room. Hospitals in developing countries also may lack certain infection-control measures - such as special containers to dispose of syringes - that are standard in U.S. facilities, Osterholm says. Wearing full-body protective garments â?? commonly called "moon suits" â?? is also more of a challenge in open-air clinics, because the restrictive outfits can cause people to quickly overheat.

More help is needed from around the world, Morse said. He received an e-mail today from a friend who is treating patients in the region and working 12- to 24-hour days. "When people tell me they're working flat-out 20 hours a day, obviously more resources are needed," he said.

The region needs more health care workers, especially those well trained in infection control procedures, he said, more equipment to keep health care workers and family members safe while treating patients, and more training for the general public about how to avoid and cope with the virus. "With something this size, it's obvious that we're under resourced right now," Morse said.

Symptoms of Ebola include fever, muscle aches, chills, sore throat, vomiting and diarrhea and a rash, according to the WHO. Advanced cases also can cause heavy bleeding, both internally and from the mouth and nose. Ebola can damage multiple organs, causing kidney and liver failure.

Only about half of patients begin hemorrhaging, Monroe says. That makes it easy for health care workers in Africa to mistake Ebola for diseases with similar symptoms, such as malaria and Lassa fever, another viral illness common in that region.

"Most people who contract Ebola are those who live with and care for those who have already caught the disease and are showing symptoms," Monroe says.

Hospitals in the USA are on high alert for Ebola, however, and would quickly isolate anyone with suspicious symptoms who has recently returned from Africa, Osterholm says.

"Right now, we'd have to assume every case is an Ebola case," in people with suspicious symptoms, Osterholm says.

In a worst-case scenario, Osterholm says, a handful of emergency room workers could be exposed before a sick person is diagnosed.

Once people are infected, however, there is no effective treatment, Osterholm says. Anti-viral medications used for other illnesses, such as the flu and HIV, don't appear to work on Ebola. Instead, hospitals could provide supportive care, dealing with symptoms as they occur.

Among the federal travel restriction procedures the Centers for Disease Control can use to protect travelers and the public from communicable diseases that pose a serious threat is to put travelers' names on a "Do Not Board" list that is enforced by the Transportation Security Administration.

However, it's unclear how well the list works. CDC still has not released records requested by USA TODAY in 2010 under the Freedom of Information Act relating to failures of the "Do Not Board" list to stop passengers from flying. As of April, CDC had transferred the gathered records for further review by officials at CDC's parent agency, the U.S. Department of Health and Human Services, according to e-mail correspondence.